A flash memory device, for example, has a configuration in which: a large number of memory cells are arrayed in the word line direction and in the bit line direction; and each memory cell is formed with an inter-electrode insulating film provided between a floating gate electrode film as a charge storage layer and a control gate electrode film. In this configuration, the flash memory device uses a structure in which the control gate electrode films CG extend downwards at element isolation regions. Thereby, the coupling between the control gate electrode films CG and the floating gate electrode films FG is enhanced. In order to further enhance the coupling, the control gate electrode films CG may extend further downwards at the element isolation regions. However, if the amount of downward extension, namely the amount of etching back, is too large, the upper portion of each of the floating gate electrode films, namely extremity, will have a more tapered and rounded shape as a result of etching. In recent years, the width of the floating gate electrode films and the width of element isolation trenches are becoming narrower along with higher integration of flash memory devices. This makes it more likely that: the extremity of each of the floating gate electrode films have a pointed shape; the electric field concentrates on the pointed portion; the leakage current increases between the floating gate electrode films and the control gate electrode films; and therefore, the write characteristics deteriorate.